In modern handheld devices for cellular communication systems (e.g. 3GPP), there is a desire to support multiple frequency bands (e.g. 3GPP LTE bands 7, 1, 2, 3, 8, 5, and 13). Due to limitations in technology and size constraints, the support of multiple frequency bands has traditionally been achieved using arrays of fixed frequency filters or duplex filters (e.g., dielectric coaxial resonator filters, SAW, BAW, FBAR) that are switched between using semiconductor switches. As used herein, the term “filter” should be understood as describing any hardware that generates a frequency selective frequency response and can discriminate between receive and transmit frequency response (e.g., greater than about 8 dB). The use of such fixed frequency filters introduces a number of limitations, however, in the design requirements of the system. For example, for each band of operation, a new set of hardware will have to be introduced. These additional hardware requirements result in the addition of filters and switches when expanding band support, with each band filter occupying additional space in the system, and each throw of the multiplex switch adding loss to the switch for each band filter.
In contrast, with recent technological development with respect to the size and performance devices of electrically tunable single resonance and multiresonance filters (e.g., RF MEMS, semiconductor-switched capacitor arrays, BST), tunable filtering systems can be used to support multiple frequency bands in a single system. Tunable systems exhibit their own problems, though. Most notably, with respect to tunable single resonance filters, the performance has not been satisfactory for some cellular systems due to the loss of the tuning resonator and the associated tradeoff between pass-band and stop-band attenuation. Conversely, tunable multiresonance filters have also been reported to have problems in that they have not been small enough, they have not had the power handling capability, or they were too complex or unrepeatable for proper integration in hand-held cellular equipment. As a result, the problem with tunable systems is to make it cost effective and small while at the same time meeting system requirements (e.g., 3GPP standards).
Furthermore, in addition to problems with the kinds of filter elements used to support of multiple frequency bands, there have also been problems with the interaction of transmitter and receiver elements in frequency division duplexing systems, with transmitter and receiver elements operating simultaneously at one or varying frequency separation. Specifically, for such frequency division duplexing, a problem known as duplex self-interference arises from the high power of the transmitter challenging the linearity of the receiver that can be setup with high gain to deal with comparatively low power reception levels.
One way to deal with these problems can be to create a spatial separation of receiver and transmitter antennas (i.e., systems where transmitter and receiver have separate antennas so some duplex isolation is created between the antennas), but such a configuration is not preferable in all systems, particularly in systems where the receiver and transmitter antennas cannot be physically spaced apart by significant distances.
Locating the transmitter and receiver antennas in close proximity introduces further equipment requirements and/or other considerations, though. For instance, for co-location of transmitter and receiver (i.e., any frequency division duplex system where the transmitter and receiver are within a certain proximity such that there can be a radiated, board, or circuit-born leakage path therebetween), a filter is required in the receive path that primarily rejects the transmit frequency to avoid overstearing or suppress intermodulation products in the receiver. For example, notch filtering at transmitter frequency in the receiver chain can be used. Similarly, a filter is required in the transmit branch that primarily rejects the transmitter noise at the receive frequency. Likewise, for co-use of the same antenna for transmitter and receiver (i.e., any system where receiver and transmitter use the same antenna element for creating a radiation), a duplex filter can be used.
In addition to providing isolation between transmission and reception signals, another issue in frequency-domain duplexing for cellular applications (e.g., LTE band application) is with bands that switch from having the reception frequency higher than the transmission frequency to having the reception frequency below the transmission frequency. For example, the majority of 3GPP standard LTE frequency-domain duplexing bands 1 to 25 have reception frequency above transmission frequency (i.e., positive duplex spacing), but bands 13, 14, 20 and 24 have the reverse order (i.e., negative duplex spacing) so that the reception frequency is below the transmission frequency. Configuring a system to allow for operation using both kinds of band spacing can further require yet additional filters and switches.
Still another issue with the location of filters in present day's phones is that filters have had a design constraint on component height of less than 1 mm so it could be placed together with RF transceiver integrated circuitry, digital processing integrated circuitry, and multimedia processing integrated circuitry.
In view of all of these problems and design considerations, it would be desirable for systems, devices, and related methods to incorporate tunable filters that can tune over a wide frequency range and at the same time minimize pass band attenuation and maximize stop band attenuation. Furthermore, it would also be desirable for such filters to change the characteristics of the filter to accommodate negative duplex spacing instead of switching between different filter hardware.